The invention describes a display device for displaying information, comprising a display unit with a plurality of columns and rows and a display driver. Further the invention describes a display driver controlling a display with storing means for providing image information to the columns of a display. The invention relates also to a terminal for mobile communication with a display device, having a display unit and a display driver.
Liquid crystal displays (LCD) are commonly used in portable computer systems, televisions and other electronic devices. LCDs have become especially popular in portable computer applications because they are sufficiently rugged and require little space to operate. Notably in applications in display devices built into portable apparatuses like mobile telephones and Personal Digital Assistants PDAs is the aim to drive these apparatuses with a minimal energy.
Since passive matrix type liquid-crystal display devices need no costly switching elements and are less expensive than active matrix liquid crystal display devices, the passive matrix type liquid crystal display devices find widespread use as monitors of portable computers and portable electronic apparatuses.
Passive matrix displays of this type are generally known and often used and, to be able to realize driving of a large number of rows, they are more and more based on the STN (Super Twisted Nematic) effect.
Many applications for LCD drivers are battery operated, most being mobile phones. Battery lifetime is one of the key market drivers for such phones. If the current consumption of such a device can be reduced then the standby time can be increased. Alternately, the battery capacity may be reduced giving a reduction in weight, another key factor. Turning the display off during standby mode is the best way to save power, however this means the user will not know if the devices is functioning or not functioning, some information must still be made available to the user. So it is required to be able to activate part of the display to show some useful information, e.g. the network provider or the time. Using part of the display is a compromise between having the display completely on or off.
The multiplex rate has a strong effect on the current consumption. LCD driver circuits use a multiplex method for driving large numbers of segment e.g. 64 rows by 100 columns, giving control over 6400 pixels. Simply the display works by selecting a row and then presenting the column data associated with that row on the column outputs. The display driver circuit then moves onto the next row and the next set of column data. This has to be repeated at sufficient rate so as not to generate flickering on the display. The number of rows driven defines the multiplex rate. When every row has been driven once, a frame is said to have taken place. Each row is only active once per frame and hence the average voltage across any one pixel VRMS is proportional to the multiplex rate. The higher the multiplex rate the lower the average voltage on the pixel. To compensate for the low average voltage, the supply voltage VOP applied to the pixel is raised. The supply voltage is generated from a DC-DC converter. Every micro-amp taken from VOP is reflected down to the supply voltage VDD with a multiplication factor equivalent to the number of stages in the DC-DC converter. Reducing the display supply voltage can make a huge current saving. When the multiplex rate is reduced, the VOP is reduced, and ultimately the current IDD is reduced also. So roughly if the number of stages is halved, the supply current IDD for display device will be halved.
The EP 0 844 600 A1 describes a liquid crystal display device (LCD) having a first display portion and a second display portion in the same panel, in addition to a normal operation mode in which both of the display portions are driven there is also provided a power-save operation mode in which only the second display portion is driven. In the power-save operation mode, the duty ratios of the display portions in the power-save operation mode are lower than those in the normal operation mode and time shared drive wave-forms are applied using an unchanged power source voltage, which does not require a bias voltage. A liquid crystal device having reduced power consumption is thus provided.
In current display devices with the possibility of a reduced active display area, the active area is hard coded into the silicon chip. This allows only the use of the certain number of rows i.e. on the top or on the bottom of the display. This is a severe limitation to the end user since it would be nice to have the possibility to display the network provider across the center of the display and perhaps some icon information along the bottom. It is possible in hardware to select any group of rows for xe2x80x98partial displayxe2x80x99 operation, but this is then fixed for the driver circuit and unsuitable as a universal LCD driver.
It is therefore an object of the invention to provide a display device and a display driver of the type described above in which the necessary drive voltage and the power consumption is as small as possible in conjunction with a freely programmable multiplex rate.
This is achieved by the display device to the claim 1 and the display driver to the claim 10.
Thus, it is possible to change the multiplex rate and the display area freely.
An advantage of the invention is the strongly decreased power consumption if the multiplex rate is lower.